CPU power/heat recycling

  OK the idea is this, people have found many ways of cooling down that hot CPU and some people have used it for positive ideas like makeing coffie of the heat from the cpu or keeping their food warm. Well what im wondering is why no one has taken the heat from the CPU and contained it so the computer stays cool while the contained heat is used to power the computer. Taking the heat from the CPU and turning it into energy that the computer can use instead of wasting all that energy that the computer is giving off.